Prosthetic knees generally include three main components, a femoral component (FIGS. 1A and 1B), which is attached to the distal end of the femur, a tibial component (FIGS. 2A and 2B), which is implanted onto the proximal end of the tibia, and an articular insert (FIGS. 3A and 3B), which is mounted onto the tibial component and provides a frictional surface for the femoral component. The components are designed to simulate a joint and the associated mechanics of a human knee throughout the knee's range of motion. The components are generally provided in a variety of shapes with varying dimensions (identified as dimensions A-H and J-T in FIGS. 1A to 3B), so that a physician is able to select the optimal combination of components depending on the specific anatomy of the patient. The size and shape of the knee is dependent on various factors including age, gender, and size of the patient. Therefore, a fairly large inventory of components are generally made available, so that the prosthetic knee may be tailored for the patient.
During the course of a routine knee construction with a TKR, the ACL is removed in a vast majority of all cases and depending on the selected TKR design, the patient's PCL is either retained or substituted with some mechanism to replace the lost function of the PCL. Even when the PCL is retained, often a portion of the PCL must be cut or partially cut during surgery to aid in the balancing of the knee replacement. When the PCL is completely removed, the PCL is substituted by a post and cam mechanism.
Referring to FIGS. 1A, 1B, 3A, and 3B, an illustration of a typical design of a post and cam mechanism is provided. An articular insert 22 includes an extension 24 that protrudes into an opening 12 of the femoral component 10. The extension 24 includes a posterior surface 25 that is intended to be in frictional contact with the posterior surface 14 of the opening 12 when the joint is flexed. The resistance generated when the extension 24 bears against the posterior surface 14 of the opening 12 in the femoral component 10 is intended to simulate the resistance that would have been generated by a healthy posterior cruciate ligament (PCL). Cam and post mechanisms have been manufactured that partially replace the function of an ACL by creating a cam surface between the anterior surface of the extension 24 and the anterior surface of the opening 12; however, this solution provides only a partial substitution of an ACL because the anterior side of the extension 24 is at best able to contact the anterior side of the opening only between 0 to 20 degrees of flexion.
Another solution is to connect the femoral component and tibial component with a cable-like material, such as the material disclosed in U.S. Pat. No. 5,935,133, the contents of which are incorporated herein by reference. But this artificial material is typically only used to replace the PCL and not the ACL.
These common solutions are deficient in that a replacement has not been provided to generate resistance simulating an ACL. Essentially, the ACL is sacrificed. The lack of an anatomically correct replacement may result in a TKR having reduced functionality compared to the original knee. This may create difficulties during physical therapy following surgery, as well as limit the patient's ability or desire to participate in physical activity following therapy. Virtually all modern total knee replacements sacrifice the ACL or inadequately substitute it with a crude cam and post mechanism, thus leaving the reconstructed knee with kinematics similar to that of an ACL-deficient knee. Normal knee kinematics therefore remain elusive. In addition, the lack of proper interplay between an ACL and PCL (which together drive normal knee kinematics) leaves the TKR reconstruction short of producing a relatively normal knee for the patient.
Given the complexity of the mechanics of a knee joint and the difficulty for patients to adjust to an artificial knee after surgery, an anatomically correct knee replacement system is needed that more accurately simulates the resilience and support formerly provided by the removed ligaments.